Thermosensitive recording elements have wide application in the printing industry. For example, thermosensitive recording elements have been used in recording instruments for measurements such as facsimiles, printers, thermal devices for computers, devices for preparing architectural and engineering drawings, automatic vending machines for dispensing railroad tickets and luggage tags, and thermal label printing devices. Conventional thermosensitive recording elements generally comprise a support and a thermosensitive recording layer provided thereon. The thermosensitive recording layer primarily contains a binder, a substantially colorless electron donating dye precursor and an electron accepting compound also known as a developer. Upon heating, by means of a thermal head, a thermal pen or laser beam, the dye precursor instantaneously reacts with the electron accepting compound to form a recorded image.
It has been found that defects in the imaging surface of thermosensitive recording elements can be easily obtained. For example, rubbing the element gently with hard objects such as paper clips, fingernails and dirt particles or even folding the element can result in undesirable imprints in the imaging surface. This presents a barrier to their use as facsimile papers, architectural and engineering drawings, luggage tags, thermal printed labels, and the like.
Abrasion resistance can be greatly improved by separating the imaging components, i.e., the colorless dye precursor and the developer. This is achieved by applying, for example, the developer containing coating composition to a support that is first coated with the dye precursor containing composition. Although abrasion resistance is improved, the resulting heat sensitive product has been found to have a relatively low gloss as indicated by a total reflection of 30 to 50 percent of incident light and has a flat or low gloss appearance. Furthermore, due to the hydrophilic nature of the polymeric binders in the two component compositions, these coatings are subject to image development when stained with water.
Another way of improving abrasion resistance in thermosensitive recording elements is the use of high gloss coatings. The use of high gloss coatings is advantageous in thermal painted labels, tags, or material for advertising. Such coatings can be produced by adding a top coating containing film forming polymers such as polyvinylalcohol or acrylic-methacrylic copolymers over the outermost layer, for example, the developer containing layer produced by the two coating process. However, polymer containing top coats such as these reduce much of the abrasion resistance afforded by the two coating techniques, and they are subject to image development when stained with water. In addition, the low softening point of such polymers, causes the polymer to adhere to the print head of thermal printers, thereby forming images of poor quality. The additional layer also adds to the cost of the thermosensitive element.
High gloss coatings having good abrasion resistance may be produced by applying glyoxal/p-toluenesulfonic acid/polyvinylalcohol containing coatings to a support that is coated with a composition containing both dye precursor and developer in the same layer. However, such top coats tend to promote the reaction of dye precursor and developer resulting in development, and therefore, a gray color to the background areas of the coated element is produced.
Accordingly, a need exists for thermosensitive recording elements that have improved abrasion resistance without producing the shortcomings discussed above. It has been found that these conditions are met using the thermosensitive recording element of the invention.